Loop antennas comprising loop-shaped conductors, curl antennas comprising curl-shaped conductors, and other types of antennas have been designed in the past as antennas for generating circularly polarized waves.
Disclosed loop antennas include an antenna comprising a C-type loop element obtained by cutting out a predetermined gap in a portion of a loop-shaped conductor, whose length is approximately equal to 1 wavelength of the radiated circularly polarized waves, a linear I-shaped conductor one end of which is connected to the C-type loop element and the other end of which serves as a feed point, a ground plane disposed in parallel to the C-type loop element, and a feed conductor, which is connected to the feed point and transmits electric power to the feed point (for example, see Patent document 1).
In addition, disclosed curl antennas include an antenna comprising a spiral-shaped curl section with a turns number in the range of from about 1 to about 1.5, in which a wire conductor is formed by joining semi-circles of different shape and whose length is approximately equal to 1 wavelength of the radiated circularly polarized waves, and a shaft section, one end of which is connected to the beginning of the curl section and the other end of which is connected to a power supply conductor (for example, see Patent document 2).    [Patent Document 1] Japanese Patent No. 3431045    [Patent Document 2] JP H8-17289B.